If you don't already have it, download and install the
Sims2Pack Clean Installer. Before you install any new expansion packs run the clean installer and make note of any files that are marked in red. These are global mods. All of them will have to be checked in the original places you downloaded them from to make sure they are compatible with the new expansion packs you are installing. You may need to download updated versions of them.
Before installing the new expansion packs move your Downloads folder to the desktop for safe keeping. After you have installed the new expansion packs
slowly put the contents of your Downloads folder back in the Sims 2 folder. Clothing, floors, walls, and most cloned/recoloured objects will be fine to put back the way they are.
Hacked objects, generally objects with new interactions, (your "fun" objects
) will also need to be checked in the original threads for compatibility. Once you are sure you have the latest and greatest versions of your global mods and hacked objects, you can install them, too. Also, visit
www.simwardrobe.com and download the Sims 2 Hack Conflict Utility then install it. It will detect potential conflicts among the global mods and can save you a ton of pain. Good luck